This invention relates to a method for controlling the conductance of a heated cellular substrate. The invention also relates to a method for minimizing the amount of conductive material required to form the resistance heater on the end surface of a cellular substrate.
It is estimated that about 50% of the pollutants escaping into the atmosphere from automotive exhaust gases for example, is generated in the period of one to two minutes after a cold engine start-up. By reducing the light-off time, the amount of pollutants escaping into the atmosphere can be reduced. Modifications are being made to catalytic converters to increase the rate of heating in order to obtain shorter light-off times. Co-pending U.S. application Ser. No. 07/893,256, titled "Heated Cellular Substrates," by Bagley et al., filed concurrently herewith and incorporated herein by reference, discloses a method for preheating a cellular structure or honeycomb to provide an early light-off catalytic converter by forming a resistance heater directly onto the surface of the structure and providing means for heating the structure, for example, by passage of electrical current. According to the reference, the conductance of a heated cellular substrate is a function of the cell wall thickness and the thickness of the conductive material applied to the substrate. Higher conductances (lower resistances) are obtained with substrates having thicker cell walls when the cell walls are coated with a conductive material than for thin wall substrates.
Typical cell wall thicknesses for ceramic automotive catalytic converters vary from the range of 0.11 to 0.15 mm for thin wall substrates to 0.18 to 0.20 mm for the standard wall substrates. As a result, the available path for current flow in the thin cell wall substrates is approximately half of that provided by the standard wall substrates. Therefore, when the same thickness of conductive material is formed on a thin and a standard wall substrate, the conductance of the thin wall substrate is significantly less than that of the standard wall substrate.
The heater is generally formed on the end portion of the cell walls which represent only a minor percentage of the substrate surface area. Since most of the conductive material is formed at or near the end portion of the substrate, the resistance is greatly affected by the thickness of the cell walls. As the wall thickness decreases, the resistance increases significantly and conversely, the conductance decreases. Therefore, significantly higher voltages are required to heat thin walled samples.
For automotive application, it is desirable to limit the voltage requirement to less than about 27 volts (that is, the capacity of two standard car batteries.) The voltage requirement can be limited by controlling the resistance of the heater. One way of improving the conductance of coated cellular substrates is to increase the thickness of the conductive material or to apply the conductive material on thicker wall substrates. Since the metals used for heated substrates generally include platinum, rhodium, silver, palladium and other rare and expensive metals, it is desirable to minimize the amount of such metals used for making heated substrates. Accordingly, it is the object of the present invention to provide a method for controlling the conductance of heated substrates. It is also the object of the invention to provide a method for minimizing the amount of conductive material needed to form the conductive layer on such substrates.